Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
; CCAST, World Lab, Beijing 100080, Peoples R China
; Chinese Acad Sci, Inst Theoret Phys, Beijing 100080, Peoples R China
The Poynting vector, energy density, and energy velocity of light pulses propagating in an anomalous dispersion medium with two closely spaced gain lines are calculated. The results show that a negative energy density in the medium propagates along the opposite direction of incidence with a velocity similar to the negative group velocity v. while Poynting vector is in the same direction of incidence. In other words, one might say that a positive energy density in the medium would propagate along the positive direction with a speed \v(g)\. We further point out that neither energy velocity nor group velocity is a good concept to describe the propagation process of light pulse inside the medium in the Wang, Kuzmich, and Dogaiu [Nature London) 406, 277 (2000)] experiment owing to the strong accumulation and dissipation effects.